Making phosphoric acid of high purity



Patented Jan. 4, 1944 UNITED STATES PATENT orrlca MAKING PHOSPHORIC ACID HIGH John Barry Coleman, Warren Township, Somerset County, and Stephen G. Poll, Woodhridge, N. J., assignors to Westvaco Chlorine Products Corporation, New York, N. Y., a corporation of Delaware No Drawing.

Application November 9, 1940, Serial No. 365,127

9 Claims. (Cl. 23-105) available CaO and P205 in an over-all ratio lower than the 1:1 ratio of calcium metaphosphate, Ca(P03)z (or 0202205), the ratio being advan-. tageously from 3:4 to 1:2, said mixture usually containing more or less gangue carrying acidsoluble compounds of iron and alumina and also containing fluorine, and the mixture is heated in a current of hot gases to a temperature of about 350 C. with removal of moisture and fluorine compounds, thereby forming a dry intermediate containing available CaO and P205 in the desired ratio, and this intermediate is treated with strong sulfuric acid of a concentration adapted to liberate phosphoric acid principally as the meta acid, and to form CaSOr in a stable form readily filtering and washing and containing water of crystallization in an amount substantially less than that of gypsum, suflicient water being added to the system to give a liquidphase containing relatively concentrated phosphoric acid, and said liquid phase is separated from the solids, advantageously by filtering, to obtain directly a strong marketable phosphoric acid solution of high purity; all as more fully hereinafter set forth and as claimed.

Phosphoric acid and phosphates have generally been made heretofore from the calcium orthophosphates (including fluophosphates) which occur in rock phosphate, apatite, pebble phosphate and other crude conunercial phosphatic materials. In the usual wet methods, these ores are digested with relatively weak aqueous sulfuric or phosphoric acids or mixtures thereof to produce an impure dilute solution of orthophosphoric acid' or monocalcium orthophosphate or mixtures thereof. The solutions thus obtained are generally quite dilute, seldom having a density above 25 to 35 B., and have therefore required evaporative concentration to obtain a material of marketable strength. These solutions also require purification to eliminate the so-called metallic impurities (mainly iron and alumina compounds) prior to sale or utilization. Both the concentration and the purification of the crude solutions obtained by these prior processes involve difiiculties well known in the art, and the procedures are costly.

An object achieved by the present invention is the provision of a simple anddirect method of making phosphoric acid of high purity and high concentration, which avoids any need for evaporative concentration and which permits a greatly simplified separation of the recovered phosphoric acid from the gangue and metallic impurities and fluorine, and from calcium sulfate formed in the process. These results are achieved in accordance with this invention by taking advantage of the discovery that suitably prepared compounds of lime and P205 containing a lower ratio of 02.0 to P205 (that is, more P205) than calcium metaphosphate, Ca(P03)z, can be acidified to produce phosphoric acid largely in the metaphosphoric form; and that under these conditions the usually troublesome impurities,

such as iron and alumina, remain with the gangue in a form readily separable from the phosphoric acid by simple filtering, along with the calcium sulfate produced by acidification. Furthermore, the fluorine which usually accompanies crude phosphates is largely eliminated in this process, going forward with the oil gases.

The phosphoric acid obtained in accordance with this invention meets the requirements of a food grade" phosphoric acid without any further purification.

In the practice of this invention, a preliminary step is the production of a calcium phosphate which is more acid than monocalcium metaphosphate in the sense that it contains a higher pro-' with this invention; and best removal of these impurities is. obtained when the initial step results in compounds or mixtures of the'type represented by the formula CaaPaOzs, in which the ratio of lime to P205 is 3:4. g

Many methods and starting materials are available .for producing combinations of lime and phosphoric acid having the desirable CaO:Pa05 ratio of 3:4; and numerous other compounds having even higher contents of P205 can also be produced in various ways. Among these compounds is CaP4On, or CaO.2P2O5, in which the ratio of lime to P205 is 112. Such complexes or polyphosphates having compositions indicated by empirical formulas ranging from CazPaOzz to CaPsOn (that is, those containing lime and P205 fire I,

compounds of iron and alumina. It is advantain ratios between 3:4 and 1:2) are especially desirable for this invention. Compositions containing higher proportions of P205 generally form so much free metaphosphoric acid under the conditions of operation that a pasty condition develops on fumacing, making the manipulation more difllcult. And as previously noted, compositions containing higher proportions of lime give less purification from metallic or acid-soluble impurities. When the lime ratio is higher than 1:1, substantially no purification is obtained.

solid mixtures can be obtained in this step, in

spite of the high P205 content.

In one typical procedure, the starting materials are crude phosphate rock (which may be considered to be essentially a fluorine-containing tricalcium orthophosphate with more or less gangue), and crude phosphoric acid. The reaction involved may be indicated by the following equation:

1. Ca3(PO4)2+6HaPO4+heat (350 C.) =CaaPaO23+9H2O It should be understood that the existence of CasPaoza as a definite compound has not been fully established, and that this equation is merely intended to indicate the proportions of CaO and P205 in the product, which can also be desi nated 3CaO.4P2Os.

The water formed by this reaction is driven off by the heating, and a dry granular material is obtained. A substantial proportion of the fluorine occurring in the crude rock is also eliminated by heating, and the complex calcium polyphosphate obtained by the stated reaction is quite low in fluorine. It is advantageous to grind the rock rather fineso that 95 per cent will pass through a 100-mesh screen, for .example--prior to the reaction. The reaction may be effected in any desired type of apparatus, an internally heated rotary kiln being generally most convenient.

. The .complex calcium phosphate material re- ,sultingfrom this procedure consists essentially of .the stated 3CaO.4P2O5 composition, sometimes edacid calcium metaphosphate, and gangue the. starting material, which usually carries geously cooled, and is then charged into an acid- .resistant mixing tank or chamber where it is treated fwith strong sulfuric acid-advantageously having a density of 66 B. (93 per cent H2304 by weight). The complex acid phosphate should be maintained in contact with the sulfuric acid at a temperature of the order of 150 C. for about one hour, in order to obtain the bestresults. During this time, free phosphoric acid is liberated, largely in the meta condition, and calcium sulfate is formed. This calcium sulfate carries substantially less water of hydration than gypsum, and generally less than the hemi-hydrate, but is nevertheless in a relatively stable form which filters and washes readily and does not rehydrate appreciably during washing. It is probably a mixture of anhydrite with one or more of the hydrated forms of calcium sulfate.

Iron and alumina in the gangue are apparently converted into metaphosphates by this treatment, and in any event are not greatly attacked by the sulfuric acid or the liberated phosphoric acid. The principal reaction may be expressed by the equation:

The acidulated material is discharged as a dry or semi-fluid paste, advantageously while still hot, into a tank provided with a suitable acidproof lining and an agitator. In this tank it is mixed with wateradvantageously strong wash water from a previous batchwhich is supplied in such proportions that a concentrated solution of phosphoric acid having a density of about 50 to 55 B. is formed. As noted, the acid as liberated is substantially all in the meta condition, but it soon reverts to pyrophosphoric acid and orthophosphoric acid by combination with the water present during this extraction stage.

An outstanding advantage of this procedure is that, as noted,-the iron and alumina and other metallic impurities present in the starting material are largely converted to insoluble metaphosphates during the thermal treatments, and are thus readily separated from the phosphoric acid solution. Another advantage is that the calcium sulfate formed is largely in a stable crystalline form, which is readily filtered and washed. A simple filtering operation separates the pure phosphoric acid from the calcium sulfate and gangue constituents. and relatively small quantitles of wash water are required in recovering retained acid from the filter cake.

Instead of starting with crude rock phosphate in the manner described, various other sources of P205 may be employed in accordance with this invention. For example, a beneficiated phosphatic intermediate containing P205 in the form of dicalcium pyrophosphate may be employed. Such an intermediat is readily produced by heating crude phosphate rock with crude phosphoric acid at a temperature of about 270 C., as described and claimed in a copending application Serial No. 365,126, filed November 9, 1940. This calcium pyrophosphate intermediate has a very low fluorine content, since more than per cent of the original fluorin i eliminated during its production. The substitution of this intermediate for crude rock in the process of the present invention therefore makes it possible to obtain a final product of especially high purity. The reaction involved in forming the so-called acid calcium metaphosphate material of the present invention, when starting with the calcium pyrophosphate intermediate, may be illustrated by the following equation:

In addition to the fact that this invention provides an improved method of producing phosphoric acid of high purity, it also provides a method of purifying crude phosphoric acid previeusly produced by any method. For example, the acid employed in accordance with Equation 1 and 3 may be crude acid obtained in accordance with the process of my above-identified application. When this crude acid is combined with lime, which is usually associated with further P205 values, to form the acid calcium metaphosphate containing lime and P205 in ratios between 1:1 and 1:2 in accordance with this invention. and then acidified with strong H2804, a phosphoric acid of high purity and desirable concentratlon is produced without resorting to expensive purifying procedures or evaporative concentration.

In a useful embodiment of this invention, crude phosphate rock was'ground to such finenes that about 95 per cent would pass through a IOO-mesh screen, and was mixed with crude Orthophcsphoric acid in such proportions that the mixture contained available lime and P205 in a molar ratio of 2:1. This mixture was charged to a rotary internally fired kiln where it was heated to a temperature of about 270 C. for about 25 minutes, thus producing an intermediate containing substantially all its phosphatic values in theiorm of dicalcium pyrophosphate, Ca2P2O7, in accordance with the acknowledged application Serial No. 365,126, filed November 9, 1940. A substantial part of this calcium pyrophosphate intermediate was then mixed with 60 sulfuric acid (78 per cent H2304) and strong wash water from a prior batch, to form crude phosphoric acid, initially in the Dy o form, which reverted to orthophosphoric acid in the presence of the water. third to one-half of the crude acid thus obtained was back-cycled to treat further quantities of phosphate ore to obtain a further quantity of the calcium pyrophosphate intermediate.

The remaining portion of the calcium pyrophosphate intermediate of low fluorine content from the first operation was mixed with a sumcient quantity of the crude orthophosphoric acid from the second operation, to give the mixture a contained ratio of available lime and P205 of slightly less than 3:5. This mixture was heated to a temperature of 380 0., with a total heating time of 3 hours, thus forming a dry granular product containing substantially all of the phosphate in the form of a complex calcium polyphosphate containing more P205 than mono-calcium metaphosphate. This material was cooled to about 150 C. and mixed with about 3 molar equivalents of concentrated sulfuric acid, then agitated with strong wash water from a previous batch of pure acid. The slurry thus obtained was filtered on a vacuum filter, and a good yield of phosphoric acid solution having a density of 55 B. was obtained.

This acid contained only 0.2 per cent of combined iron and alumina (based on the P205 content) and, less than 18 parts fluorine per million parts of P205. The purity of the acid thus obtained will readily be understood to be exceptional for processes of this type involving no separate purification. Also the concentration is notable in processes employing no evaporative treatment. These features contribute to the economy of the process; and the ease with which the calcium sulfate and gangue are filtered out, due to the relatively dense and stable condition of the C8504, also contribute to this result. As stated, the calcium sulfate formed contains less water of crystallization than gypsum (that is, less than 2H20 per molecule of calcium sulfate), and often About oneless than the Ob -molecule of H20 found in hemihydrate. Also, this sulfate is considerably denser than gypsum; it neither locks up so much water as water of crystallization nor does it occupy so much space. Relatively little wash water is required in this process because of the condition in which the calcium sulfate precipitates; and the wash water which is produced is readily utilized in the process as described hereinabove. It is, of course, desirable to utilize the wash water from the crude acid filtration in the production of further quantities of the crude acid, while the Wash water from the final pure acid filtration is advantageously kept separate and utilized in the production of further quantities of pure acid.

It is well known that most phosphatic raw materials contain considerable amounts of associated gangue, which usually includes calcium carbonate and other calcium minerals acted on by sulfuric acid. Also, the usual fluorine content oi" raw phosphates is probably due to calcium fluoride in apatite-like combination with tri-calcium orthophosphate. The lime in such compounds must generally be consideredas available CaO in computing overall'ratios of Ca0:P20s for the purposes of this invention.

The term available CaO whenever used herein refers to calcium (computed as CaO) in a form reactive with the digesting acids in contradistinction to calcium that may be present in a form non-reactive with the digesting acids ,which must be considered as unavailable CaO, therefore to be excluded in computing the ratio Ca0:P20s. For example, calcium silicate may be prcsent'in the phosphatic raw materials in a form not reactive with the digesting acid. Such calcium silicate will go forward and be discarded in what is called gangue. Other acid insoluble compounds containing calcium in a form unavailable, such as calcium sulphate may also be present. If the calcium in these acid insoluble compounds is taken into consideration in calculating the Ca0:P205 ratio it is obvious that an excessive amount of phosphoric acid would be used. Calcium present in available" form will react with the sulfuric acid used and be removed as precipitated calcium sulphate.

It is to be understood that the terms CaO, P205 and alumina," as used in the present specification and claims, do not necessarily refer to individual and discrete compounds. 0n the contrary, in the process disclosed these entities probably exist as components of complex salts or the like. While the exact nature of all the chemical compounds in rock phosphate is not specifically known, these expressions may be used in specifying the practice of the present invention.

Similarly the term phosphoric acid is broadly employed and contemplates acids containing P205 in a form available for reaction with metals or other compounds.

While the invention has been described hereinabove with special reference to certain manipulative procedures, reagents, concentrations, temperatures, etc., which are now-considered advantageous, it should be understood that it may be modified and otherwise'practiced within the scope of the appended claims.

By the term rock phosphate as used in the present specification. and claims is meant raw phosphatic materials used in the manufacture of phosphoric acid such as apatite, phosphate rock, pebble phosphate and the like. 2

vWhat we claim is:

1. In the manufacture of. phosphoric acid from phosphatic material containing CaO, P205,

gangue and fluorine and including aluminum and iron impurities. the process which comprises mixing the said phosphatic material with a phosphoric acid in such an amount that the molar ratio of the content of calcium calculated as CaO present in a chemical form reactive with phosphoric-acid and sulfuric acid, to the P205 .content in the mixture is less than 1:1 but not substantially less than 1:2, heating the mixture at a temperature from about 300 to 400 C. so

that the iron and alumina in the mixture 'are substantially converted to insoluble ironand aluminum meta-phosphates and the fluorine is substantially completely driven oil, mixing the resultant material with strong sulfuric acid so that free phosphoric acid and dense readily flltering magma containinggangue and stable calcium sulfate carrying less water than gypsum and containing insoluble iron and aluminum meta-phosphates is formed, and filtering the material to recover relatively pure, strong phos- I but not substantially less than 1:2, heating the mixture at a temperature from about 300 to 400 C. so that the said other acid soluble metal compounds in the mixture as impurities are substantially converted to insoluble metaphosphates, treating the resultant material with strong sulfuric acid to form free phosphoric acid and a dense readily separable magma containing gangue, and stable calcium sulfate carrying less water than gypsum, and containing the said insoluble meta-phosphates, and separating the phosphoric acid from the magma as a relatively pure product of strong concentration.

.3. In the manufacture of phosphoric acid from phosphatic material comprising compounds containing CaO, P205 and fluorine together with impurities comprising other acid soluble metal compounds and gangue, the process which comprises mixing the said phosphatic material with a phosphoric acid in such an amount that the molar ratio of the content of calcium calculated as CaO present in a chemical form reactive with phosphoric acid and sulfuric acid, to the P205 content in the mixture is less than 1:1 but not substantially less than 1:2, heating the mixture at a temperature from about 300 to 400 C. so

that the said other acid soluble metal compounds .phosphoric acid from the magma as a relatively pure product of strong concentration.

4. In the manufacture of phosphoric acid from phosphatic material comprising gangue and compounds containing CaO, P205, and fluorine and including acid soluble compounds of iron and aluminum as impurities, the process which comprises mixing the said phosphatic material with a'phosphoric acid in such an amount that the molar ratio of the content of calcium calculated as CaO present in a chemical form reactive with phosphoric acid and sulfuric acid, to the P205 content in the mixture is less than 1:1 but not substantially less than 1:2, heating the mixture at a temperature from about 300 to 400 C. so that the iron and aluminum in the mixture are substantially converted to insoluble iron and aluminum meta-phosphates and the fluorine is substantially completely driven off, mixing the resulting material with strong sulfuric acid so that free phosphoric acid and dense readily flltering magma containing gangue and stable calcium sulfate carrying less water than gypsum and containing iron and aluminum meta-phosphates is formed, adding suiiicient water to dissolve the liberated phosphoric acid and form a solution thereof having the desired concentration, and filtering the material to recover relatively strong phosphoric acid.

5. In the manufacture of phosphoric acid from phosphatic material containing dicalcium pyrophosphate and gangue and including aluminum and iron impurities, the process which comprises mixing the said phosphatic material with a phosphoric acid in such an amount that the molar ratio of the content of calcium calculated as CaO present in a chemical form reactive with phosphoric and sulfuric acid, to the P205 content in the mixture is less than 1:1 but not substantially less than 1:2, heating the mixture at a temperature frcm'about 300 to 400 C. so that the iron and aluminum in the mixture are substantially converted to insoluble iron and aluminum meta-phosphates and the-fluorine is substantially completely driven off, mixing the resulting material with strong sulfuric acid so that free phosphoric acid and dense readily fil tering magma containing gangue and stable calcium sulfate carrying less water than gypsum and containing iron and aluminum meta-phosphates is formed and filtering the material to recover relatively strong phosphoric acid.

6. In the manufacture of phosphoric acid from phosphatic material comprising gangue and compounds containing CaO, P205 and fluorine together with acid soluble compounds of aluminum and iron as impurities, the step which comprises treating a mixture of said phosphatic material and added ortho-phosphoric acid wherein the molar ratio of the content of calcium calculated as 08.0 present in a chemical form reactive with phosphoric acid and sulfuric acid, to the P205 content in the mixture is less than 1:1 but not substantially less than 1:2 and which has been heated to a temperature of 300 to 400 C. until the iron and aluminum compounds are converted into insoluble meta phosphates and the fluorine is substantially completely driven off, with strong sulfuric acid to form a free phosphoric acid and a dense readily separable magma containing gangue and stable calcium sulfate carrying less water than ypsum and containing insoluble iron and aluminum meta-phosphates, and separating the phosphoric acid from the magma,

'7. A process as defined in claim 2 wherein the mixture is heated at a temperature within the range of 350 to 380 C.

8. A method as defined in claim 2 wherein the mixture of phosphatic material with phosphoric acid is in such relative amount that the molar ratio of CaOzPzOa is within the range of from about 3:4 to about 1:2. 7

9. In the manufacture of phosphoric acid from phosphatic material comprising gangue and compounds containing CaO, P205 and fluorine together with acid soluble compounds of aluminum and iron as impurities, the step which comprises treating a mixture of said phosphatic material and added ortho-phosphoric acid wherein the molar ratio of the content of calcium calculated as CaO present 10 perature of 300 to 400 C. until the iron and 15 aluminum compounds are converted into insoluble meta-phosphates and the fluorine is substantially completely driven off, with strong sulfuric acid to form a free phosphoric acid and a dense readily separable magma containing gangue and stable calcium sulfate carrying less water than gypsum and containing insoluble iron and aluminum meta-phosphates, adding suflicient water to dissolve the liberated phosphoric acid and form a solution thereof having the desired concentration and separating the solution from the magma. I

JOHN HARRY COLEMAN.

STEPHEN G. POLL.

CERTIFICATE OF CORRECTION. Patent No. 2, ,L;o3. 7 January A, 19%.

JOHN HARRY coIEMANmT AL.

It is hereby certified that error. appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, first column, line 1+6, for {'less than 5:5" read --less than 3:11.; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this lBth day of April, A. D. 19%.

Leslie Frazer (Seal) Acting Commissioner of Patents. 

